Laser diode operable in 1.3 μm or 1.5 μm wavelength band with improved efficiency

ABSTRACT

A laser diode includes an active layer of a group III-V compound semiconductor device containing N and As as the group V elements. The active layer has exposed lateral edges wherein the N atoms are substituted by the As atoms at the exposed lateral edges by an annealing process conducted in a AsH 3  atmosphere.

BACKGROUND OF THE INVENTION

The present invention generally relates to optical semiconductor devicesand more particularly to an optical semiconductor device operable in a1.3 μm or 1.5 μm wavelength band.

Today, a telecommunication trunk generally uses an opticaltelecommunication system in which optical fibers carry informationtraffic in the form of optical signals. Currently, quartz glass opticalfibers having an optical transmission band of 1.3 μm or 1.5 μmwavelength are used commonly. In correspondence to the foregoingspecific transmission band of the optical fibers, current opticaltelecommunication systems generally use a GaInAsP double-heterojunctionlaser diode that includes an active layer of In_(1-x) Ga_(x) As_(y)P_(1-y) and a cladding layer of InP. In such a GaInAsPdouble-heterojunction laser diode, the carriers are accumulated in theactive layer by a potential barrier formed in the conduction band andthe valence band between the GaInAsP active layer and the InP claddinglayer, and stimulated emission of photons is substantially facilitatedin the active layer by the carriers thus accumulated therein. In orderto obtain a laser oscillation at the wavelength that matches the opticaltransmission band of the quartz glass optical fibers, the compositionalparameter x for Ga and the compositional parameter y for As are adjustedappropriately.

However, such a conventional laser diode that uses adouble-heterojunction of GaInAsP and InP has suffered from the problemof relatively large threshold current of laser oscillation and poortemperature characteristic, primarily due to the relatively small banddiscontinuity (ΔEc) of the conduction band between the GaInAsP activelayer and the InP cladding layer. More specifically, the electronsescape easily from the active layer in such an GaInAsP laser diodebecause of the small potential barrier ΔEc formed by the foregoing banddiscontinuity, and a large drive current has to be supplied in order tosustain a laser oscillation in the active layer. This problem becomesparticularly acute at high temperatures in which the carriers experiencean increased degree of thermal excitation. Further, the foregoingGaInAsP laser diode has a problem in that the laser oscillationwavelength tends to shift to a longer wavelength side at hightemperatures due to the temperature dependence of the bandgap ofGaInAsP. It should be noted that the bandgap of GaInAsP decreases withtemperature. This shift of the laser oscillation wavelength raises aserious problem particularly in a wavelength multiplex transmissionprocess of optical signals.

In order to avoid the foregoing problems, conventional GaInAsPdouble-heterojunction laser diodes for use in optical telecommunicationtrunk or submarine optical cable systems have used a temperatureregulation device, such as a Peltier cooling device, such that theoperational temperature of the laser diode is maintained at apredetermined temperature.

On the other hand, there is a strong impetus to expand the use ofoptical telecommunication technology from the telecommunication trunksto subscriber systems or home systems. In relation to this, there is ademand for an optical semiconductor device suitable for use in hometerminals.

When realizing such optical home terminals, it is essential that theoptical home terminal is compact and low cost. Further, the optical hometerminal should consume little electric power. In order to meet suchdemands, it is necessary to provide a laser diode that is operable inthe 1.3 or 1.5 μm band with a low threshold current and simultaneouslywithout a temperature regulation.

As long as the foregoing GaInAsP/InP double-heterojunction system isused, the foregoing demand cannot be satisfied. Thus, efforts are beingmade to construct a laser diode having an active layer of GaInAs on aGaAs substrate such that a large band discontinuity ΔEc is secured inthe conduction band. By increasing the In content in the GaInAs activelayer, it is possible to reduce the bandgap energy Eg of the activelayer, and the oscillation wavelength of the laser diode approaches thedesired 1.3 μm band. However, such an increase of the oscillationwavelength by increasing the In content in the GaInAs active layer issuccessful only to the point in which the oscillation wavelength reachesabout 1.1 μm. Beyond that, the lattice misfit between the GaInAs activelayer and the GaAs substrate becomes excessive and the epitaxial growthof the GaInAs active layer on the GaAs substrate is no longer possible.It should be noted that the foregoing limit of 1.1 μm takes intoconsideration the contribution of compressive strain that acts in thedirection to increase the oscillation wavelength of the laser diode.

In view of the foregoing situation, Japanese Laid-Open PatentPublication 7-193327 proposes a laser diode operable in the 1.3 or 1.5μm band, in which an active layer of GaInAs is sandwiched by a pair ofcladding layers having a composition set such that a large banddiscontinuity ΔEc is secured between the active layer and the claddinglayer and that the cladding layer has simultaneously a lattice constantclose to that of a strained buffer layer provided on a GaAs substratewith a composition of Ga₀.8 In₀.2 As. However, the proposed device isdeemed to be unrealistic in view of the large lattice misfit between theactive layer and the GaAs substrate. It is believed that the existenceof such a large lattice misfit reduces the lifetime of the laser diodesubstantially.

On the other hand, Japanese Laid-Open Patent Publication 6-37355describes a compound semiconductor structure that includes a GaInNAsmixed crystal film formed on a GaAs substrate. By adding N to GaInAs, itbecomes possible to form the GaInNAs film with a lattice constant thatmatches the lattice constant of GaAs. The GaInNAs film thus added with Nhas a reduced bandgap due to a large negative bowing of thebandgap-composition relationship observed in a GaAs--GaN system. Thus,it is expected that a double-heterostructure laser diode having anoscillation wavelength in the 1.3 or 1.5 μm and simultaneously a largeband discontinuity ΔEc necessary for carrier accumulation, may beobtained by using GaInNAs for the active layer. As the GaInNAs film canhave a composition that establishes a lattice matching with GaAs, it ispossible to use an AlGaAs or GaAs cladding in combination with theactive layer of GaInNAs.

FIG.1 shows the compositional change of a bandgap Eg for a GaAs-GaNsystem according to the Japanese Laid-open Patent Publication 6-37355.

Referring to FIG. 1, it will be noted that the endmember component GaNhas a very large bandgap Eg of about 3.5 eV, contrary to the endmembercomponent GaAs, of which bandgap Eg is only about 1.4 eV. Thus, GaN isexpected to be one of the most promising materials of an active layerfor an optical semiconductor device that is operable in a blue orultraviolet wavelength band.

The striking feature of FIG. 1 is that the compositional change of thebandgap Eg between GaAs and GaN is not linear but there appears a verysignificant negative bowing. Probably, this large negative bowing ofbandgap is related to the existence of a very large difference in theatomic radius between As and N. In fact, there is reported a largemiscibility gap in the GaAs--GaN system.

Thus, a bandgap Eg as small as about 1.2 eV is possible for a GaNAssystem by incorporating N into a GaAs crystal with a proportion of about10 mole %. While the GaNAs system of this composition has a smalllattice constant due to the small atomic radius of N, a satisfactorylattice matching can be achieved, with respect to a GaAs substrate, byincorporating In.

FIG. 2 shows the construction of a laser diode 1 proposed in theJapanese Laid-Open Patent Publication 7-154023, op cit.

Referring to FIG. 2, the laser diodel1 is constructed on a substrate 10of n-type GaAs and includes a lower cladding layer 11 of n-type GaInPprovided on the GaAs substrate 10. On the lower cladding layer 11, anactive layer 12 of undoped GaInNAs is provided, and an upper claddinglayer 13 of p-type GaInP is provided further on the active layer 12,wherein the upper cladding layer 13 is formed with a ridge structureextending in an axial direction of the laser diode. Further, a pair ofcurrent confinement structures 14 of n-type GaAs are provided at bothlateral sides of the ridge structure, and a contact layer 15 of p-typeGaAs is provided on the ridge structure so as to bury the currentconfinement structures 14 underneath. Further, a p-type ohmic electrode16 is provided on the contact layer 15, and an n-type ohmic electrode 17is provided on the bottom surface of the substrate 10.

In operation, holes are injected into the active layer 12 from theelectrode 16 via the contact layer 15 and the ridge structure of theupper cladding layer 13, wherein the active layer 12 is further injectedwith electrons from the electrode 17 via the substrate 10 and the lowercladding layer. Thereby, a stimulated emission of photons occur in theactive layer 12 as a result of recombination of the electrons and holesthus accumulated in the active layer 12, and the laser diode oscillatesat the wavelength of 1.3 μm or 1.5 μm corresponding to thecharacteristically increased bandgap of the active layer 12.

In the laser diode of FIG. 2, the current confinement structures 14restricts the current path of the holes by establishing a p-n junctionbetween the current confinement structure 14 and the contact layer 15,wherein it should be noted that each current confinement structure 14 isformed above the active layer 12, with a part of the upper claddinglayer 13 intervening between the current confinement structure 14 andthe active layer 12.

While it is usual in conventional laser diodes to form a currentconfinement structure corresponding to the current confinement structure14, such that the current confinement structure reaches the lowercladding layer 11 or to the substrate 11, across the active layer 12,such a construction, when applied to the active layer 12 having thecomposition of GaInNAs, would cause a problem of extensive defectformation at the lateral edges of the active layer 12 where the activelayer 12 is laterally bounded by the GaAs current confinement structures14. It should be noted that such a formation of the current confinementstructure includes a mesa formation step for forming the ridgestructure, while the etching process used in such a mesa formation steptends to introduce a substantial amount of defects into thesemiconductor layer thus is subjected to the etching process. The activelayer 12 containing N therein is particularly susceptible to defects, inview of rapid deteriorating crystal quality with increasing content of Nin the crystal.

In the device of FIG. 2, the problem of defect formation and associatednon-optical recombination of carriers is successfully avoided by formingthe current confinement structures 14 above the active layer 12 with aseparation therefrom. On the other hand, such a construction naturallyallows a lateral diffusion of the injected holes away from the regionimmediately under the ridge structure as indicated by arrows in FIG. 2.Thereby, the threshold current of laser oscillation increases and theefficiency of laser oscillation is deteriorated substantially.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful laser diode wherein the foregoing problems areeliminated.

Another and more specific object of the present invention is to providea laser diode operable in a 1.3 or 1.5 μm wavelength band wherein theefficiency of laser oscillation is improved.

Another object of the present invention is to provide a laser diodehaving an active layer of a group III-V compound semiconductor materialcontaining N as a group V element, wherein a ridge stripe is formed by amesa structure extending in an axial direction of the laser diode suchthat a mesa wall reaches below a level of the active layer, and whereinthe problem of non-optical recombination of carriers at the lateraledges of the active layer is successfully eliminated.

Another object of the present invention is to provide an edge-emissiontype laser diode, comprising:

a substrate of a first conductivity type;

a lower cladding layer of said first conductivity type provided on saidsubstrate;

an active layer of an undoped group III-V compound semiconductormaterial containing therein N and As as group V elements;

an upper cladding layer of a second, opposite conductivity type providedon said active layer;

a first ohmic electrode provided on said upper cladding layer forinjecting carriers of a first polarity into said active layer via saidupper cladding layer; and

a lower ohmic electrode provided on a lower major surface of saidsubstrate for injecting carriers of a second, opposite polarity intosaid active layer via said substrate and said lower cladding layer;

at least said active layer and said lower cladding layer forming a mesastructure laterally defined by a pair of mesa walls and extending in anaxial direction of said laser diode,

said active layer having first and second lateral edges exposedrespectively at said pair of mesa walls, each of said first and secondlateral edges of said active layer having an modified composition and acorrespondingly increased bandgap.

According to the present invention, the active layer of the laser diodecontains a small amount of N as a group V element, in addition to As.Thereby, the laser diode oscillates at the optical wavelength of 1.3 or1.5 μm. Further, the active layer of the laser diode is laterallydefined by the first and second lateral edges, wherein the bandgap isincreased at the first and second lateral edges as compared with aregion inside the active region. Thereby, the carriers injected into theactive layer do not reach the lateral edges, and the problem ofnon-optical recombination of the carriers, caused by defects included insuch exposed laterals edges, is successfully avoided.

Another object of the present invention is to provide a vertical-cavitysurface-emitting laser diode, comprising:

a substrate;

a lower multilayer reflection structure provided on said substrate;

a lower cladding layer of a first conductivity type provided on saidlower multilayer reflection structure;

an active layer of an undoped group III-V compound semiconductormaterial containing therein N and As as group V elements, said activelayer being provided on said lower cladding layer;

an upper cladding layer of a second, opposite conductivity type,provided on said active layer;

an upper multilayer reflection structure provided on said upper claddinglayer;

an upper ohmic electrode for injecting carriers of a first polarity intosaid active layer; and

a lower ohmic electrode for injecting carriers of a second, oppositeconductivity type into said active layer;

at least said active layer and said lower cladding layer forming astructure defined by a pair of side walls on said lower multilayerreflection structure,

said active layer having first and second lateral edges exposedrespectively at said pair of side walls, each of said first and secondlateral edges of said active layer having an modified composition and acorrespondingly increased bandgap.

According to the present invention, too, the active layer of thevertical-cavity surface-emitting laser diode contains a small amount ofN as a group V element, in addition to As. Thereby, the laser diodeoscillates at the optical wavelength of 1.3 or 1.5 μm. Further, theactive layer of the laser diode is laterally defined by the first andsecond lateral edges, wherein the bandgap is increased at the first andsecond lateral edges as compared with a region inside the active region.Thereby, the carriers injected into the active layer do not reach thelateral edges, and the problem of non-optical recombination of thecarriers is successfully avoided.

Another object of the present invention is to provide a method offabricating an optical semiconductor device operable in a 1.3 μm or 1.5μm wavelength band, comprising the steps of:

forming a lower cladding layer on a substrate;

forming an active layer of a group III-V compound semiconductor devicecontaining N and As as a group V element, on said lower cladding layer;

forming a mesa structure including at least said active layer and saidlower cladding layer, such that said mesa structure is defined by a pairof mesa walls and such that said active layer is exposed at said mesawall; and

substituting N atoms in an exposed part of said active layer by Asatoms.

According to the present invention, the N atoms in the exposed lateraledges of the active layer are replaced by the As atoms and a widegapmaterial is formed in the active layer in correspondence to the exposedlateral edges. Thereby, the problem of non-optical recombination ofcarriers at such exposed lateral edges of the active layer issuccessfully eliminated.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a bandgap composition relationship of aGaAs--GaN system;

FIG. 2 is a diagram showing the construction of a conventional laserdiode operable in a 1.3 μm or 1.5 μm band;

FIG. 3 is a diagram showing the construction of an edge-emission typelaser diode according to a first embodiment of the present invention;

FIGS. 4A-4C are diagrams showing the fabrication process of the laserdiode of FIG. 3; and

FIG. 5 is a diagram showing the construction of a vertical-cavitysurface-emitting laser diode according to a second embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows the construction of an edge-emission type laser diode 100according to a first embodiment of the present invention.

Referring to FIG. 3, the laser diode 100 is constructed on a substrate101 of n-type GaAs and includes a buffer layer 102 of n-type GaAsprovided on the substrate 101 epitaxially with a thickness of typicallyabout 500 nm. On the buffer layer 102, a lower cladding layer 103 ofn-type AlGaAs is provided epitaxially with a thickness of typicallyabout 1500 nm. Further, a lower optical guide layer 104 of undoped GaAsis provided on the lower cladding layer 103 epitaxially with a thicknessof typically about 100 nm, and an active layer 105 of undoped GaInNAs isprovided on the lower optical guide layer 104 epitaxially with athickness of typically about 10 nm.

The active layer 105 has a composition of Ga_(x) In_(1-x) N_(y) As_(1-y)and achieves a lattice matching with the GaAs substrate 101. Further,the active layer 105 has a bandgap of 0.95 eV corresponding to theoptical wavelength of 1.3 μm or 0.82 eV corresponding to the opticalwavelength of 1.5 μm.

On the active layer 105, an upper optical waveguide layer 106 of undopedGaAs is provided epitaxially with a thickness of about 100 nm, and ap-type AlGaAs layer 107 of the p-type is provided further on the upperoptical waveguide layer 106 with a hickness of about 500 nm as a part ofthe upper ladding layer to be formed.

It should be noted that the layers 103-107 form together a mesastructure defined by sloped lateral walls and extending in an axialdirection of the laser diode, and a current blocking layer 109 of p-typeAlGaAs and another current blocking layer 110 of n-type AlGaAs aredeposited consecutively at each lateral side of the mesa structure.

Further, an upper cladding layer 111 of p-type AlGaAs and a contactlayer 112 of p-type GaAs are provided consecutively on the p-type AlGaAslayer 107, and an upper ohmic electrode 113 of an AuZn/Zn stackedstructure is provided on the contact layer 112. Further, a lower ohmicelectrode 114 of an AuGe/Ni/Au stacked structure is provided on thelower major surface of the substrate 101.

In the structure of FIG. 3, it should be noted that the active layer 105of GaInNAs is formed by an MOVPE process that uses a nitrogen compoundsuch as dimethylhydrazine (DMHy) as the source of N. Further, the laserdiode 100 is axially defined by a pair of mirrors M that form an opticalcavity. The mirrors M may be formed of cleaved surfaces, as usual in theart of laser diode.

It should be noted that the mesa structure exposes both lateral edges ofthe active layer 105 as a result of the mesa formation. Because of theetching process that is used in the process of forming the mesastructure, the exposed lateral edges of the active layer tend to includesubstantial defects, while such defects tend to act as a non-opticalrecombination center. Thereby, a substantial amount of drive current,injected from the electrodes 113 and 114, is consumed at suchnon-optical recombination centers, without contributing to the photonemission.

Thus, in order to avoid the foregoing problem, the present embodimentforms a region 108 where the N atoms are substituted by the As atoms. Inother words, the region 108 of the active layer 105 has a composition ofGaInAs. Thereby, the region 108 has a bandgap substantially larger thanthe bandgap inside the active layer 105, and the carriers injected tothe active layer 105 are confined laterally by the regions 108. In otherwords, the carriers avoid the non-optical recombination centers at thelateral edges of the active layer 105, and the efficiency of laseroscillation is improved substantially.

The laser diode of FIG. 3 has the so-called SCH (separate confinementheterostructure) -SQW (single quantum well) structure in which theactive layer 105 is vertically sandwiched by the GaAs layers 104 and 106between the cladding layers 103 and 107. By using GaAs for the layers104 and 106 rather than using AlGaAs, which is used for the claddinglayers 103 and 107, the problem of non-optical recombination of carrierscaused by Al, is successfully avoided. Further, the current blockinglayers 109 and 110 not only act to confine the carriers in the mesastripe but also the optical radiation produced in the active layer 105as a result of stimulated emission.

As already noted, the laser diode of FIG. 3 operates in the 1.3 μm or1.5 μm wavelength band, wherein the laser diode has a prolonged lifetimedue to the excellent lattice matching of the active layer 105 to theGaAs substrate 101. As there is formed a large band discontinuity in theconduction band between the active layer 105 and the cladding layer 102or 111, the carriers are confined efficiently in the active layer 105even at high temperatures, and a high efficiency of laser oscillation ismaintained even when the laser diode is operated at high temperatures.

FIGS. 4A-4C show the fabrication process of the laser diode 100 of FIG.3.

Referring to FIG. 4A, a layered structure including the foregoingsemiconductor layers 102-107, is formed on the substrate 101 asdescribed already with reference to FIG. 3. The layers 102-107 may bedeposited in a reaction vessel of a deposition apparatus by an MOVPEprocess, wherein the deposition of the GaInNAs layer 105 may beconducted by using DMHy as noted already. During the deposition of theGaInNAs layer, the internal pressure of the reaction chamber istypically set to 1.3×10⁴ Pa, and the deposition may be made at asubstrate temperature of 630° C. by supplying TMG, TMIn, DMHy and AsH₃(arsine) as respective gaseous sources of Ga, In, N and As.

Next, in the step of FIG. 4B, a mesa etching is applied to the structureof FIG. 4A by a wet etching process while using a resist pattern 115 asan etching mask. Thereby, a mesa structure 116 is formed from thelayered structure of FIG. 4A. It should be noted that the mesa etchingreaches the lower cladding layer 103, and the lateral edges of theactive layer 105 is exposed at the mesa side walls.

Next, in the step of FIG. 4C, the structure of FIG. 4B is annealed,after removing the resist mask 115, in an AsH₃ atmosphere at atemperature of 630° C. for 30 minutes. Thereby, the N atoms in theactive layer GaInNAs 105 are substituted at the exposed lateral edges bythe As atoms, and the regions 108 of GaInAs are formed as a result ofsuch a substitution. Each of the regions 108 has a very limited size andcovers only the surface of the foregoing lateral edges. The illustrationof FIG. 3 or FIG. 4C is substantially exaggerated for facilitating theunderstanding.

After the step of FIG. 4C, the layers 109-112 are deposited by an MOVPEprocess as is well known in the art of stripe laser diode having a BH(buried-hetero) structure.

In the foregoing process, it should be noted that the annealing of FIG.4C for forming the GaInAs regions 108 is conducted in the samedeposition apparatus used for depositing the current blocking layers 109and 110, by merely changing the gas composition supplied to the reactionchamber. Thereby, the increase of fabrication cost of the laser diode bysuch an annealing process is successfully avoided.

In the device of FIG. 3, it should be noted that the current blockinglayers 109 and 110 may be formed of GaInPAs including InGaP, with acomposition set to form a lattice matching with the GaAs substrate 101.When the current blocking layers 109 and 110 are to be formed of GaInP,the AsH₃ atmosphere used in the step of FIG. 4C for forming the GaInAsregions 108 is switched to a PH₃ (phosphine) atmosphere when depositingthe GaInP current blocking layers 109 and 110. Further, the currentblocking layers 109 and 110 may be formed of GaInPAs, by supplying amixture of AsH₃ and PH₃ after the foregoing annealing step.

It should be noted that the foregoing process of forming the GaInAsregions 108 is not limited to the SBR laser diode explained withreference to FIG. 3, but is effective also in a general stripe laserdiode including the device in which the formation of the currentblocking layers 109 and 110 is not made.

Next, a vertical-cavity surface-emitting laser diode 200 according to asecond embodiment of the present invention will be described withreference to FIG. 5.

Referring to FIG. 5, the vertical-cavity surface-emitting laser diode200 is constructed on a substrate 201 of n-type GaAs, on which amultilayer reflector 202 is formed in the form of an alternaterepetition of a first epitaxial layer of n-type AlGaAs and a secondepitaxial layer of n-type GaAs. Typically, the first and secondepitaxial layers are repeated for 24 times in the multilayer reflector202, and a lower cladding layer 203 of n-type AlGaAs is provided on themultilayer reflector 202 epitaxially with a thickness of typically about500 nm.

On the lower cladding layer 203, a lower optical waveguide layer 204 ofundoped GaAs is provided epitaxially with a thickness of typically about100 nm, and an active layer 205 of undoped GaInNAs is provided furtheron the optical waveguide layer 204 epitaxially with a thickness oftypically about 10 nm.

On the active layer 205, an upper optical waveguide layer 206 of undopedGaAs is provided epitaxially with a thickness of typically about 100 nm,and an upper cladding layer 207 of p-type AlGaAs is provided on theupper optical waveguide layer 206 epitaxially with a thickness oftypically about 500 nm. Further, a contact layer 208 of p-type GaAs isprovided on the upper cladding layer 207 epitaxially with a thickness ofabout 100 nm.

The layers 203-208 are then subjected to a dry etching process, to forma layered body 250 on an optical axis of the laser diode to be formed,wherein the layered body 250 is defined by substantially vertical sidewalls, and a part of the top surface of the lower multilayer reflector202 is exposed as indicated in FIG. 5. Further, ohmic electrodes 211having an AuGe/Ni/Au stacked structure are provided on the exposed topsurface of the lower multilayer reflector 202.

On the contact layer 208, an upper multilayer reflector 209 is formed asan alternate repetition of an undoped GaAs layer and an undoped AlGaAslayer in alignment with the optical axis of the laser diode, and upperohmic electrodes 212 of a Cr/Au stacked structure is provided on theexposed top surface of the contact layer 208.

In the vertical-cavity surface-emitting laser diode of FIG. 5, it shouldbe noted that the side walls of the layered body 250 expose lateraledges of the active layer 205, and regions 210 having a modifiedcomposition of GaInAs are formed at such exposed lateral edges of theactive layer 205, by substituting the N atoms by As atoms similarly tothe first embodiment. Similarly to the previous embodiment, the regions210, having an increased bandgap as compared with the rest of the activelayer 205, expel the carriers penetrating thereinto, and the problem ofnon-optical recombination of the carriers by the defects in the regions210, is successfully avoided. The region 210 may be formed by carryingout a thermal annealing process in an AsH₃ atmosphere conducted at atemperature of about 630° C. for 30 minutes, similarly as before. Theannealing is preferably made before forming the upper multilayerreflector 209 or the electrodes 211 and 212.

It should be noted that the vertical-cavity surface-emitting laser diodeof FIG. 5 oscillates efficiently at the wavelength of 1.3 μm or 1.5 μmband while successfully using GaAs for the substrate 201. As a result ofthe use of GaAs for the substrate 201, it becomes possible to form thelower and upper multilayer reflectors 202 and 209 by the stacking a GaAslayer and an AlGaAs layer as already noted. Thereby, a very large changeof refractive index is induced in the reflection structures as comparedwith the conventional case of using InP for the substrate 201, and thenumber of repetitions of the GaAs/AlGaAs stacking in the lower or uppermultilayer reflectors is reduced to one-half or less. When InP is usedfor the substrate 201, the upper or lower multilayer reflectors has tobe formed by an alternate stacking of InP and InGaAsP, while such aconstruction can provide a refractive index change of only 0.25. Byusing the alternate stacking of GaAs and AlGaAs, with the composition ofAlGaAs layer set coincident to AlAs, a refractive index change of asmuch as 0.5 can be achieved. Thereby, the throughput of fabrication ofthe laser diode is improved substantially and hence the cost of thedevice.

In addition to the foregoing advantageous optical feature, thevertical-cavity surface-emitting laser diode of FIG. 5 has anadvantageous feature of excellent carrier accumulation in the activelayer 210 due to the large band discontinuity ΔEc in the conduction bandbetween the active layer 210 and the cladding layer 203 or 207. Thereby,the laser diode oscillates efficiently even at high temperatures,without providing an external cooling device, and the cost of the laserdiode is reduced substantially.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

What is claimed is:
 1. An edge-emission type laser diode, comprising:asubstrate of a first conductivity type; a lower cladding layer of saidfirst conductivity type provided on said substrate; an active layer ofan undoped group III-V compound semiconductor material containingtherein N and As as group V elements; an upper cladding layer of asecond, opposite conductivity type provided on said active layer; afirst ohmic electrode provided on said upper cladding layer forinjecting carriers of a first polarity into said active layer via saidupper cladding layer; and a lower ohmic electrode provided on a lowermajor surface of said substrate for injecting carriers of a second,opposite polarity into said active layer via said substrate and saidlower cladding layer; at least said active layer and said lower claddinglayer forming a mesa structure laterally defined by a pair of mesa wallsand extending in an axial direction of said laser diode, said activelayer having first and second lateral edges exposed respectively at saidpair of mesa walls, each of said first and second lateral edges of saidactive layer having a modified composition and a correspondinglyincreased bandgap.
 2. An edge-emission type laser diode as claimed inclaim 1, wherein said active layer has a composition of GaInNAs, andwherein said lateral edges of the active layer has a composition ofGaInAs.
 3. An edge-emission type laser diode as claimed in claim 1,wherein said active layer has said modified composition only on bothlateral side walls thereof coincident to said mesa walls.
 4. Anedge-emission type laser diode as claimed in claim 1, wherein saidsubstrate is formed of GaAs, and wherein said active layer has acomposition of Ga_(x) In_(1-x) N_(y) As_(1-y).
 5. An edge-emission typelaser diode as claimed in claim 1, further comprising current blockinglayers provided at both lateral sides of said mesa structure.
 6. Anedge-emission type laser diode as claimed in claim 1, further comprisinga lower optical waveguide layer between said lower cladding layer andsaid active layer, said edge-emission type laser diode furthercomprising an upper optical waveguide layer between said active layerand said upper cladding layer, each of said upper and lower opticalwaveguide layers having a bandgap smaller than a bandgap of any of saidupper and lower cladding layers.
 7. An edge-emission type laser diode asclaimed in claim 6, wherein said upper and lower optical waveguidelayers are formed of undoped GaAs.